The present invention relates to color picture tubes having improved inline electron guns, and particularly to an improvement in such guns for correcting astigmatism formed by a focus lens or for balancing an overfocusing caused by a deflection yoke.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths to a point or small area of convergence near the tube screen. In one type of inline electron gun, shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members havng bottoms facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29 mm tube neck. Because of this size requirement, the three focusing lenses are very closely spaced from each other, thereby providing a severe limitation on focus lens design. It is known in the art that the larger the focus lens diameter, the less will be the spherical aberration which restricts the focus quality.
In addition to the focus lens diameter, the spacing between focus lens electrode surfaces is important, because greater spacing provides a more gentle voltage gradient in the lens, which also reduces spherical aberration. Unfortunately, greater spacing between electrodes beyond a particular limit (typically 1.27 mm) generally is not permissible because of beam bending from electrostatic charges on the neck glass penetrating into the space between the electrodes, which causes electron beam misconvergence.
In U.S. Pat. No. 4,370,592, issued to R. H. Hughes and B. G. Marks on Jan. 25, 1983, an electron gun is described wherein the main focus lens is formed by two spaced electrodes. Each electrode includes a plurality of apertures therein, equal to the number of electron beams, and also a peripheral rim, with the peripheral rims of the two electrodes facing each other. The apertured portion of each electrode is located within a recess set back from the rim. The effect of this main focus lens is to provide the gentle voltage gradient sought to reduce spherical aberration. However, the main focus lens causes a slot effect astigmatism that is corrected in the electron gun by the addition of a horizontal slot opening at the exit of the second focus and accelerating electrode. This slot is formed by two parallel strips, which provide a similar effect on all three electron beams.
An improvement in the design of such a slot is disclosed in U.S. Pat. No. 4,388,553, issued to H.-Y. Chen on June 14, 1983. In this patent, the ends of two parallel strips that form the slot are tailored to create a weaker stigmator effect on the two side beams than on the center beam.
Although these prior art stigmator slots have proven very effective in correcting astigmatism, they still require the two strips, i.e., additional parts, as well as extra labor for their attachment to the electron gun. Therefore, there is a need for other means for correcting astigmatism which do not require additional parts and the associated labor required to attach those parts to an electron gun.